Luckily, I’ve had an sb66 (albeit ally) so am in the perfect place to advise 🙂
The 5 is lighter, so it s not quite as planted, but the geo is very similar so it does feel very confident on descents. There’s nothing I wouldn’t do on the 5 that I would have done on the 66.
I had mine with a 140mm fork, although you can go right up to a 160 I believe, but 140 always felt enough for me.
I had 160 on the 66 and found it too much, so I reckon 150 on the sb5 would be perfect.
I don’t think I ever felt that the 5 was out of its depth on any of the rocky stuff I did. Raced it at a fair few enduros too, and it was great!